Insulin-like growth factors (IGFs) are mitogenic peptide hormones. There are two kinds of IGFs, namely, IGF-I and IGF-II. These two polypeptides have high homology of protein folding structure and play important regulatory roles in growth and differentiation of vertebrates.
The gene structure of IGF-I has been reported in mammals (e.g., humans, rats, sheep) and fish (e.g., salmon). The mature form of the IGF-1 peptide is a 70 amino acid polypeptide which mediates the growth-promoting actions of growth hormone as well as having important local paracrine and autocrine roles in multiple organs (Kavsan et al. (1993), DNA and Cell Biology, 12:729-737).
The gene structure of IGF-II has been analyzed in various mammals including humans. Until recently, there has been no report relating to the gene structure of IGF-II in fish. The inventors of the present invention are the first to have discovered the IGF-II gene structure in fish (Chen et al., DNA and Cell Biology (1997), 16:883-892). Their findings demonstrate that the gene structure of fish IGF-II is different from that in mammals. For example, human IGF-II gene consists of 10 exons about 30 kb in length, which encode a mature, circulating polypeptide approximately 70 amino acids in length (Gray et al. (1987), DNA, 6:283-295). In contrast, the mature form of fish IGF-II polypeptide is contained in 4 exons about 13 kb in length, although it is also approximately 70 amino acids in length (Chen et al., DNA and Cell Biology (1997), supra).
The mature form of IGF-II polypeptide in different fish species is highly conserved, suggesting that an IGF-II polypeptide derived from one fish species may display similar growth promotion effects on another fish species.
Although IGF-I and IGF-II share high homology of protein folding structure and similar growth promotion effects, their biologic effects are mediated by different IGF receptors. The IGF-I receptor is a tyrosine kinase receptor; the IGF-II receptor is a mannose-6-phosphate receptor.
Recently, studies regarding the findings of fish IGF-I promoters have been reported by several groups of investigators (Koval et al. (1994), DNA and Cell Biology, 13:1057-1062; Kulik et al. (1995), J. Biol. Chem., 270:1068-1073). These reports show that the IGF-I promoters have potent stimulatory activity in cells, suggesting that the IGF-I promoters may play a regulatory role in stimulating the expression of IGF-I in cells or tissues by providing cells or tissues with transcription factor binding, especially of the liver-specific transcription factor.
So far, no study, other than the one to be presented by the inventors in the present invention, has been reported concerning the IGF-II promoter region(s) in fish.
In the invention to be presented below, the inventors will describe their findings of the DNA sequences of the IGF-II promoters in fish. Because these inventors also have completed substantial research on the IGF-I promoters and because IGF-I and IGF-II have been regarded as possessing similar regulatory functions, the inventors will present their findings of IGF-I promoters together with those of IGF-II promoters in the following sections, particularly for comparison purpose. These inventors will show that fish IGF-II promoters not only affect the growth of fish embryos earlier than IGF-I promoters but also display higher levels of gene expression than IGF-I promoters. The inventors will also show that an IGF-II promoter from one fish species not only can be expressed in eukaryotic cells other than fish, such as human cells, but also can be integrated into the somatic and germ cells of another fish species, thus creating a transgenic fish.